The invention pertains generally to electronic air/fuel ratio management systems and is more particularly directed to a calibration correction for such systems based upon the pressure differential between the exhaust manifold and intake manifold of the engine.
Electronic air/fuel ratio management systems have been developed whereby the quantity of fuel to be ingested into the intake manifold of an internal combustion engine is calculated from the measurement of various engine operating parameters. These parameters generally describe the mass air flow into the engine and primarily include the speed of the engine, the intake manifold absolute pressure and the air temperature. Other secondary parameters such as special calibrations for warm up conditions or for closed loop operation further comprise the engine coolant temperature and the composition of the exhaust gases in the exhaust manifold of the engine.
All the measured parameters are input into an electronic control unit which schedules the fuel quantity accordingly and produces an air/fuel ratio control signal. In one of the more widely used systems the air/fuel ratio control signal is provided by a pulse width signal having a variable duration. This pulse width signal, the duration of which is determined by the calculated or scheduled fuel quantity, is generated by a pulse width generation circuit of the electronic control unit at a cyclic rate dependent upon the speed of the engine. An injection apparatus or other fuel metering device responsive to the variable duration pulses of the ECU is then utilized to input the desired quantity of fuel into the engine.
An example of an advantageous air/fuel ratio management system of this type is described in an application U.S. Ser. No. 918,291 filed on June 22, 1978 in the name of Ralph W. Carp et al. which is commonly assigned with the present application. The disclosure of Carp et al. is hereby expressly incorporated by reference herein.
Generally, these air/fuel ratio management systems are used to advantage for the regulation of the air/fuel ratio of a spark ignited four-cycle internal combustion engine. In the operation of the conventional four-cycle internal combustion engine having intake, compression, power and exhaust cycles, an air/fuel ratio charge is input through an intake manifold into a cylinder where it is combusted, and the waste products output from the cylinder through an exhaust manifold. Control of the four separate timed cycles is accomplished by the opening and closing of intake and exhaust valves for each cylinder in a timed relationship.
During the closing of the exhaust valve and the opening of the intake valve, there is some valve overlap wherein both the intake valve and the exhaust valve are for a very short period open at the same time. Because of the higher pressure in the exhaust manifold than in the intake manifold during this overlap some of the exhaust gas will be recirculated into the next incoming air/fuel ratio charge. This constitutes an internal exhaust gas recirculation (EGR) wherein the leakage of noncombustible waste gases dilutes the incoming air/fuel ratio into a richer charge than the electronic control unit believes it has scheduled. This is because not all the fuel input into the cylinder can be combusted with the extra amount of nonburnable exhaust gases.
Generally the amount of internal leakage is relatively small and the valve overlap fairly constant for a specified engine configuration. Until recently this internal EGR has not posed a substantial problem to air/fuel ratio control because other sources of air/fuel ratio error tended to mask its effect. With the advent of precision electronic controllers this air/fuel ratio error is now one that can and should be compensated. It would therefore be advantageous to compensate the air/fuel ratio management system in proportion to the dilution produced by the internal EGR.
It has been found that the amount of internal EGR leakage varies primarily as a function of the changes in pressure in the exhaust manifold and the intake manifold at a constant engine speed. It would therefore be desirable to provide the internal EGR compensation as a function of the pressure changes in the intake and exhaust manifolds.
The pressure changes in these two manifolds however can not be described simply as each may contain variable restrictions to flow which change with the various operating conditions or parameters of the engine. Common exhaust manifold restrictions found today are catalytic converters and noise suppressors that have different flow characteristics at different temperatures and humidity conditions. Further, the exhaust manifold pressure will increase nonlinearly with the speed of an engine even with a fixed restriction.
The most common variable intake manifold restrictions are the throttle plate which regulates the air flow to the engine, the automatic choke used at different engine temperature settings, and any filtering apparatus placed in series with the manifold such as a common air cleaner.
Further the absolute pressures found in the intake manifold and exhaust manifold will vary with the altitude of engine operation and cause different amounts of internal EGR at different throttle positions. As the intake manifold is throttled ambient pressure changes usually affect the exhaust manifold absolute pressure more significantly than the intake manifold absolute pressure.
Generally, modern electronic control units contain an altitude compensation circuit for the density changes caused by shifts in altitude. An exemplary circuit of this type is found in the Carp et al. reference and includes compensation for EGR drop out with increasing altitude.
Another major variable restriction found on more automotive systems today is the turbocharger. Generally, such a system comprises a turbine placed in a restricting manner in the exhaust gas flow which is mechanically coupled to a rotor or air pump for providing boost pressure to the intake manifold. These restrictions are variable for both the intake and exhaust manifold as they change nonlinearly with respect to the rotational velocity at which they are operating.
For example, a stalled or stationary turbine exhibits a much greater restriction to exhaust gas flow than when the turbine is rotating. Further, if a waste gate type of turbocharger system is used wherein the turbine is always rotating, the variable waste gate will produce a variable restriction based upon an operator setting.
Particularly, on rapid accelerations the exhaust manifold pressure of a turbocharger system will not initially follow the pressure change in the intake manifold caused by the opening of the throttle. The lag in equalizing the pressure changes between the manifolds as the turbine builds up to speed may cause significant air/fuel ratio error.
All of the above mentioned restrictions provide variable amounts of internal EGR which dilute the incoming air/fuel ratio charge from that calculated to produce an air/fuel ratio error for the engine. The amount of internal EGR or air fuel ratio error on an absolute scale is directly related to the pressure differential between the two manifolds. Assuming the valve overlap remains substantially constant, it would be highly advantageous to utilize this parameter to correct the calibration for the error.